The present invention relates to a light-emitting device employing light emission from an organic electroluminescent device and a display device employing a plurality of such light-emitting devices by which mixture of light emitted by adjacent organic electroluminescent devices is avoided and thereby light extraction efficiency (efficiency in extracting light from the light-emitting devices to outside) is improved.
An organic electroluminescent device (which will hereinafter be called xe2x80x9corganic EL devicexe2x80x9d) is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. Since a report on a low-voltage-driven organic EL device employing multilayered structure was released by C. W. Tang et al. of Eastman Kodak Co. (C. W. Tang, S. A. VanSlyke, Applied Physics Letters, Vol.51, page 913 (1987)), extensive researches have been made on organic EL devices, that is, EL devices employing organic materials.
In the above report, an organic EL device employing tris(8-quinolinol)aluminum complex for the light-emitting layer and triphenyldiamine derivative for the hole transporting layer was fabricated on a glass substrate. The multilayer structure has some advantages such as: improved hole injection to the light-emitting layer; increase of production efficiency of excitons which are generated by recombination (by blocking the paths of electrons injected from the cathode); and confinement of the excitons generated in the light-emitting layer.
As the structure of the organic EL devices, two-layer types (including a hole transporting (and injection) layer and an electron transporting light-emitting layer) and three-layer types (including a hole transporting (and injection) layer, a light-emitting layer and an electron transporting layer) are well known. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
Further, organic EL devices involve certain limitations on the probability of the creation of singlet excited states of light-emitting material molecules on carrier recombination since the carrier recombination is dependent on spin statistics, thereby the probability of light emission is necessitated to have an upper limit. The upper limit is known as approximately 25%.
Furthermore, in organic EL devices, rays of light whose outgoing angles (getting out of the light-emitting layer) are larger than a critical angle (depending on the refractive index of the light-emitting material) can not get out of the light-emitting layer due to total reflection. Therefore, when the refractive index of the light-emitting material is 1.6, only about 20% of the total light emission is available outside, and the upper limit of energy conversion efficiency becomes as low as approximately 5% taking the singlet excited state creation probability into account (Tetsuo Tsutsui xe2x80x9cPresent situation and trends in organic electroluminescencexe2x80x9d, Display (monthly), vol.1, No.3, page 11 (September 1995). In organic EL devices, having tight limitations on the light emission probability, low light extraction efficiency (low efficiency in extracting light from the organic EL device to outside) causes fatal deterioration of the (total) luminescent efficiency.
Methods for improving the light extraction efficiency have been studied so far for light-emitting devices of similar structure such as inorganic EL devices. For example, in Japanese Patent Application Laid-Open No. SHO 63-314795, the light extraction efficiency is improved by forming or attaching light convergent optics on substrate, which is effective for devices having large light-emitting areas. However, in light-emitting devices whose pixel areas are small (such as dot matrix displays), the formation of lenses for light convergence is difficult.
In another method disclosed in Japanese Patent Application Laid-Open No. SHO 62-172691, an anti-reflection coating is formed by providing a flat layer of an intermediate refractive index between the substrate glass and the light-emitting layer, thereby the light extraction efficiency to the front is improved considerably. However, the aforementioned total reflection can not be eliminated by the method. Therefore, while being effective for inorganic EL devices (including materials with large refractive indices), the method can not effectively improve the light extraction efficiency of organic EL devices (including light-emitting materials of relatively low refractive indices).
In order to reduce the total reflection at a surface of the substrate opposite to the organic EL device, Japanese Patent Application Laid-Open No.2000-323272 disclosed a technique for providing the substrate surface with a function for diffusing light. However, the technique can not achieve a marked effect for the conventional glass substrates since the rate of total reflection of light at the interface between the organic EL device and the glass substrate is high. Further, if a light-emitting device having a plurality of organic EL devices arranged in a matrix is manufactured using such a substrate having the light-diffusing function, light emitted by an organic EL device (corresponding to a pixel area) tends to reach adjacent pixel areas, thereby the light leakage problem (light emission from pixel areas that are not supposed to emit light) occurs.
In order to resolve the light leakage problem, Japanese Patent Application Laid-Open No. HEI11-8070 disclosed a method, in which a black mask and a light diffusing layer were formed between the substrate and the organic EL devices. However, the method further deteriorates the light extraction efficiency, since part of the light emitted by the organic EL devices is absorbed by the black mask.
As described above, the prevention of light leakage and the improvement of light extraction efficiency in light-emitting devices employing organic EL devices are both still insufficient. Especially in the method of Japanese Patent Application Laid-Open No. HEI11-8070, the light extraction efficiency (main problem to be resolved) is sacrificed for the prevention of light leakage. Therefore, techniques capable of satisfying both of the requirements are being sought for, and the development of such techniques is essential for practical utilization of the organic EL devices.
It is therefore the primary object of the present invention to provide a light-emitting device and a display device of high performance by preventing the light leakage and improving the light extraction efficiency of the light-emitting device including an organic EL device.
In accordance with a first aspect of the present invention, there is provided a light-emitting device comprising an organic electroluminescent device including one or more organic thin layers at least including a light-emitting layer which are sandwiched between a transparent first electrode formed on a light-transmitting substrate and a second electrode. In the light-emitting device, the light-transmitting substrate at least includes reflection means for reflecting light emitted by the organic electroluminescent device corresponding to a pixel and thereby preventing the light from entering adjacent pixel areas, and the light-transmitting substrate has a refractive index of 1.65 or more.
In accordance with a second aspect of the present invention, in the first aspect, in a cross section taken along a plane perpendicular to both a reflecting surface of the reflection means and the light-transmitting substrate, the height h of the reflection means measured from a contacting surface of the light-transmitting substrate with the first electrode is set so as to satisfy hxe2x89xa7Hxc2x74t/(1+3t). Here, t=d/D, D: the distance between the centers of two reflection means surrounding the organic electroluminescent device, d: the distance between two reflecting surfaces of the two reflection means facing the organic electroluminescent device, and H: the distance between the contacting surface and an opposite surface of the light-transmitting substrate.
In accordance with a third aspect of the present invention, in the first aspect, a surface of the light-transmitting substrate opposite to the first electrode is provided with light-diffusing means for diffusing the light emitted by the organic electroluminescent device.
In accordance with a fourth aspect of the present invention, in the third aspect, in a cross section taken along a plane perpendicular to both a reflecting surface of the reflection means and the light-transmitting substrate, the height h of the reflection means measured from a contacting surface of the light-transmitting substrate with the first electrode is set so as to satisfy hxe2x89xa7Hxc2x74t/(1+3t). Here, t=d/D, D: the distance between the centers of two reflection means surrounding the organic electroluminescent device, d: the distance between two reflecting surfaces of the two reflection means facing the organic electroluminescent device, and H: the distance between the contacting surface and an opposite surface of the light-transmitting substrate.
In accordance with a fifth aspect of the present invention, in the first aspect, a surface of the light-transmitting substrate opposite to the first electrode is provided with a second light-transmitting substrate having a refractive index of 1.65 or more and having light-diffusing means for diffusing the light emitted by the organic electroluminescent device on its surface opposite to the light-transmitting substrate.
In accordance with a sixth aspect of the present invention, in the fifth aspect, the refractive index of the second light-transmitting substrate is set higher than that of the light-transmitting substrate.
In accordance with a seventh aspect of the present invention, in the fifth aspect, the light-transmitting substrate and the second light-transmitting substrate are formed of resin.
In accordance with an eighth aspect of the present invention, in the seventh aspect, the light-emitting device further comprises a gas barrier layer which is provided between the light-transmitting substrate and the second light-transmitting substrate.
In accordance with a ninth aspect of the present invention, in the eighth aspect, the gas barrier layer is formed of material having gas barrier properties against H2O and/or oxygen.
In accordance with a tenth aspect of the present invention, in the eighth aspect, the light-emitting device further comprises a third light-transmitting substrate having a refractive index of 1.65 or more which is provided between the gas barrier layer and the light-transmitting substrate.
In accordance with an eleventh aspect of the present invention, in the first aspect, the reflection means is formed by metal that is embedded in the light-transmitting substrate.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, metal whose work function is 4.3 eV or less is employed as the metal embedded in the light-transmitting substrate.
In accordance with a thirteenth aspect of the present invention, in the eleventh aspect, the metal embedded in the light-transmitting substrate is formed so as to function also as an assist electrode for the first electrode.
In accordance with a fourteenth aspect of the present invention, in the thirteenth aspect, metal whose work function is 4.3 eV or less is employed as the metal embedded in the light-transmitting substrate.
In accordance with a fifteenth aspect of the present invention, there is provided a display device comprising: a plurality of first lines arranged in the same direction; a plurality of second lines arranged perpendicular to the first lines; the light-emitting devices of the first aspect which are placed corresponding to a plurality of pixel areas of the display device which are formed in a matrix of rows and columns by the first lines and the second lines; a voltage control circuit for controlling the voltages of the first lines and the second lines; common power supply lines for supplying driving currents to the light-emitting devices; and thin-film transistors each of which is provided corresponding to each pixel, for receiving a video signal at its gate and controlling the conduction between the common power supply line and the organic electroluminescent device.
In accordance with a sixteenth aspect of the present invention, in the fifteenth aspect, the reflection means of the light-emitting devices are implemented by arranging a plurality of combshaped metal parts, and each of the combshaped metal parts is formed so as to function also as an assist electrode for the first electrode.
In accordance with a seventeenth aspect of the present invention, in the fifteenth aspect, the reflection means of the light-emitting devices are implemented by arranging a plurality of metal parts having through-holes, and each of the metal parts having the through-holes is formed so as to function also as an assist electrode for the first electrode.
In accordance with eighteenth through twenty-ninth aspects of the present invention, there are provided display devices employing the light-emitting devices of the second through fifth aspects, similarly to the fifteenth, sixteenth and seventeenth aspects.
In accordance with thirtieth through forty-third aspects of the present invention, there are provided light-emitting devices having the same basic structure as the light-emitting devices of the first through fourteenth aspects. Instead of the light-transmitting substrate, the second light-transmitting substrate and the third light-transmitting substrate having refractive indices of 1.65 or more, a light-transmitting substrate, a second light-transmitting substrate and a third light-transmitting substrate having refractive indices higher than that of the light-emitting layer are employed.
In accordance with a forty-fourth aspect of the present invention, there is provided a display device comprising: a plurality of first lines arranged in the same direction; a plurality of second lines arranged perpendicular to the first lines; the light-emitting devices of the thirtieth aspect which are placed corresponding to a plurality of pixel areas of the display device which are formed in a matrix of rows and columns by the first lines and the second lines; a voltage control circuit for controlling the voltages of the first lines and the second lines; common power supply lines for supplying driving currents to the light-emitting devices; and thin-film transistors each of which is provided corresponding to each pixel, for receiving a video signal at its gate and controlling the conduction between the common power supply line and the organic electroluminescent device.
In accordance with a forty-fifth aspect of the present invention, in the forty-fourth aspect, the reflection means of the light-emitting devices are implemented by arranging a plurality of combshaped metal parts, and each of the combshaped metal parts is formed so as to function also as an assist electrode for the first electrode.
In accordance with a forty-sixth aspect of the present invention, in the forty-fourth aspect, the reflection means of the light-emitting devices are implemented by arranging a plurality of metal parts having through-holes, and each of the metal parts having the through-holes is formed so as to function also as an assist electrode for the first electrode.
In accordance with forty-seventh through fifty-eighth aspects of the present invention, there are provided display devices employing the light-emitting devices of the thirty-first through thirty-fourth aspects, similarly to the forty-fourth, forty-fifth and forty-sixth aspects.
In accordance with a fifty-ninth aspect of the present invention, there is provided a light-emitting device comprising an organic electroluminescent device including one or more organic thin layers at least including a light-emitting layer which are sandwiched between a transparent first electrode formed on a light-transmitting substrate and a second electrode. In the light-emitting device, the light-transmitting substrate has a refractive index of 1.65 or more, and a surface of the light-transmitting substrate opposite to the first electrode is provided with light-diffusing means for diffusing light emitted by the organic electroluminescent device.
In accordance with a sixtieth aspect of the present invention, there is provided a display device comprising: a plurality of first lines arranged in the same direction; a plurality of second lines arranged perpendicular to the first lines; the light-emitting devices of the fifty-ninth aspect which are placed corresponding to a plurality of pixel areas of the display device which are formed in a matrix of rows and columns by the first lines and the second lines; a voltage control circuit for controlling the voltages of the first lines and the second lines; common power supply lines for supplying driving currents to the light-emitting devices; and thin-film transistors each of which is provided corresponding to each pixel, for receiving a video signal at its gate and controlling the conduction between the common power supply line and the organic electroluminescent device.
In accordance with a sixty-first aspect of the present invention, there is provided a light-emitting device comprising an organic electroluminescent device including one or more organic thin layers at least including a light-emitting layer which are sandwiched between a transparent first electrode formed on a light-transmitting substrate and a second electrode. In the light-emitting device, the light-transmitting substrate has a refractive index higher than that of the light-emitting layer, and a surface of the light-transmitting substrate opposite to the first electrode is provided with light-diffusing means for diffusing light emitted by the organic electroluminescent device.
In accordance with a sixty-second aspect of the present invention, there is provided a display device comprising: a plurality of first lines arranged in the same direction; a plurality of second lines arranged perpendicular to the first lines; the light-emitting devices of the sixty-first aspect which are placed corresponding to a plurality of pixel areas of the display device which are formed in a matrix of rows and columns by the first lines and the second lines; a voltage control circuit for controlling the voltages of the first lines and the second lines; common power supply lines for supplying driving currents to the light-emitting devices; and thin-film transistors each of which is provided corresponding to each pixel, for receiving a video signal at its gate and controlling the conduction between the common power supply line and the organic electroluminescent device.